Rabbi Yisroel Belinow, 50, is either a fool or very brave. As I walked near a mosque, I saw him looking out of his window. His home was firebombed in 2009, and a kosher restaurant next door burned down.

He’s since watched other Jews flee the area, and his dying father begged him to leave before it was too late. He came down to speak to me, but declined my invitation to stroll around the streets.

‘My parents came here from Russia and Poland,’ he told me. ‘When I was a kid, there were the usual jokes between children; we made fun of each other, but there was always a limit. I could go anywhere I wanted whenever I wanted.’

‘The problem is people coming to France and wanting to change it. And it’s worse because they want to force people to change. I know I look different. The hatred is obvious — people spit when you walk past.

‘I respect this country because I was born here. I respect the laws of this country. I respect Christmas even though it has nothing to do with being a Jew.

‘Now they won’t let Christmas happen. France has existed for thousands of years. If I didn’t like those laws, I would move to another country.

‘It wasn’t always like this. In the beginning, [French people] wanted to help. The charity these people [recent migrants] were shown was tremendous. But you wake up and realise pretty soon that this works one way only. Many people have left.’

The scale of the problem grows each day. An estimated 80 migrants arrive in Paris every 24 hours — 550 a week.

Many head for Saint-Denis because of its closeness to transport links, including the railway lines heading towards the North coast, and Britain.

Migrant camps, set up in tents along the Seine in this area of Paris, were destroyed by the police in May, with the occupants who didn’t get away taken for processing in detention centres after a raid.

But there are still campers everywhere, as well as people sleeping on the streets.

There are an estimated 135 different nationalities in Saint-Denis, most extremely poor, including an estimated 600,000 Muslims from North African or sub-Saharan African backgrounds.

Having spent several days in Saint-Denis, it’s clear to me that the area is already lost to France — to the rule of French law, equality, religious freedom, and even access to the streets by the police themselves.

Indeed, this is a parallel state — a state within a state, with its own rules and religious courts — where allegiance to Islam comes ahead of fealty to France.

People bought and sold drugs openly. What law there is takes place inside Sharia courts, where Islamic leaders dispense the same forms of justice practised in the countries from which many here fled.

And where, as I discovered, other faiths and religions are being driven from the area.

When helicopters flew overhead in training for the Bastille Day celebrations earlier this month, one man pretended to shoot at them with a machine gun.

Another pushed him away and pretended to fire a shoulder-mounted missile, tracing the missile with his hand towards its targets and shouting: ‘Boom!’ Everyone laughed.

Further down the street, there was a flurry of activity. A woman was surrounded as she opened a huge bag full of phones, shoes, sunglasses and handbags — clearly stolen from tourists or Parisians. The goods were quickly sold and the crowd melted away.

Since then, as I discovered, in many ways the situation has worsened, although thankfully there has not been another major terrorist attack.

There are around 350 known jihadists living in Saint-Denis, while 1,700 are believed to have returned to France after fighting for IS in Syria, with 15,000 terrorism suspects in France.

In Saint-Denis itself, there is a record number of mosques — 160 official ones, and many more unofficial — compared with 117 Catholic churches and 60 Protestant. Yet it is the unauthorised mosques — set up in basements and garages — that the authorities fear the most.

‘The radicalisers use these hidden places of worship to influence the young and impressionable,’ said a veteran police officer who has worked in Saint-Denis for more than two decades.

He added: ‘Salafists (followers of an extreme form of Islam) impose the rule of religion, so we can have very little influence. These radicalisers are the ones who motivate the young towards terrorism.’

While only a few hundred attend weekly mass at the Basilica here, thousands of Muslims stream into the area’s mosques for Friday prayers — so much so that, in a rare intervention, the authorities banned them from praying in the streets as well.

Women suffer the most. Not far from the drug dealers outside the station, I visited a women’s refuge set up by Ghada Hatem, a senior gynaecologist, who says almost one in five of her patients have been victims of female genital mutilation (FGM) — the barbaric ritual of cutting the sexual organs of young women.

Now a specialist in the repair of such intimate mutilation, Hatem, who hails from Lebanon, says she is in daily contact with ‘women who tell me about the horrors they experience at home’.

Sarah Oussekine, who has an Algerian background and who runs a group called the Voix d’Elles Rebelles (Voice of the Female Rebels) in Saint-Denis, says: ‘When you ask girls why they are starting to wear the headscarf — and many more are — they tell you it is an act of faith, but actually when you dig deeper, they have to wear it to stay safe.’